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Plant mystery A previously unknown mass extinction of plants occurred around a million years ago in the southeast corner of Australia, an analysis of fossilised leaves shows.

The findings, by Australian researchers, helps explain a spectacular but mysterious diversity of sclerophyll plants elsewhere - in Australia's southwest region.

Australian Research Council Fellow Dr Kale Sniderman, of the University of Melbourne, says the diversity of sclerophyll plants in Australia's southwest, and also in South Africa's western Cape, is a global "anomaly".

"They are unusually rich in species diversity [and] have the most diverse floras [in the world] outside the tropics," he says.

Sniderman says it has long been assumed this diversity was the result of the arrival of Mediterranean-type climates to these regions.

However their study suggests otherwise. It shows that about 1.5 million years ago southeastern Australia was awash with sclerophyll flora, every bit as spectacularly diverse as the southwest corner.

Importantly, says co-author Dr Greg Jordan, at the University of Tasmania, this sclerophyll flora diversity in southeastern Australia occurred at a time when the region was much wetter and warmer than it is today.

This finding, published in today's PNAS journal , is based on an analysis of "exceptionally preserved" sclerophyll fossilised leaves, flowers and fruits from Stony Creek Basin, an infield lake near Daylesford, about 100 kilometres from Melbourne.

Sclerophyll are tough-leaved, woody plants common to Australia such as banksias, grevilleas, hakeas, acacias and eucalypts.

The researchers found leaf and stem fossils of 69 sclerophyll trees and shrubs at the site.

They then compared this find with the diversity of leaves deposited in a modern-day equivalent lake system - Lake Dobson in Tasmania.

Sniderman says by knowing how much leaf deposit in this modern-day equivalent reflected local diversity, they were able to extrapolate the diversity at Stony Creek Basin 1.5 million years ago.

'Something special'

Jordan says it was traditionally believed there was "something special" about the climate in southwestern Australia and the western Cape that allowed lots of species to flourish.

However he says the study shows an equal level of sclerophyll diversity under "a completely different climate" more than a million years ago.

"These results undermine the notion that summer-dry Mediterranean-type climates are necessary for the evolution of hyperdiverse sclerophyll diversity," the authors write.

Instead the study suggests the diversity can then be explained by the fact southwestern Australia and the western Cape did not suffer a severe loss of flora during the Ice Age.

"It looks like the size of the climate cycles between warm and wet and cold and dry [in the west] weren't as great as in eastern Australia," says Jordan. "So these were places where lots of species didn't disappear."

Sniderman says the study also has implications in understanding the evolution of the landscape.

There had been an assumption that sclerophyll flora had "chased" rainforests, into small enclaves in a process known as ecological substitution, he says.

However the study shows the loss of sclerophyll diversity in the east coincided with the decline of rainforests.

"What our results show is that sclerophylls have also undergone quite large-scale loss of species [and] are a skeleton of their former diversity," says Sniderman.